If you're in Perranporth, your right on the money with a wide choice of beaches. If you're beachcombing for interesting stuff washed up tho, the best bet is to be up early. Check the tide times online (Easytide) for next w/e and if poss, get on the shore at first light to get the best pickings. Its amazing how many dog walkers are up early!
Regarding mussels, that area should be OK if away from human or agricultural runoff. Use common sense and when cooked, discard any that don't open.
Enjoy!
Intertidal